December 19, 2012

Harvard Scientist Knows The Nose

Science is a magnificent thing and yet, there is still so much we don´t yet understand. For instance, scientists are still working to fully understand the part of our body that resides right in front of our face. More than 100 years ago, scientists discovered a mechanism that provides feedback from our nose to our brain. Though these scientists discovered this mechanism, they weren´t yet able to fully understand how it works to deliver these signals from our olfactory system to the brain - until now.

Venkatesh Murthy, a professor of Molecular and Cellular Biology and a team of three other scientists have finally been able to describe how this mechanism works. Today, they´ve published their work in a paper in the journal Neuron.

In a statement, Murthy explained their findings, saying, “The image of the brain as a linear processor is a convenient one, but almost all brains, and certainly mammalian brains, do not rely on that kind of pure feed-forward system.

"On the contrary, it now appears that the higher regions of the brain which are responsible for interpreting olfactory information are communicating with lower parts of the brain on a near-constant basis.”

As previously mentioned, scientists have long been aware of this specialized mechanism, though they didn´t know specifically how it worked. For example, until Murthy and his team began investigating this mechanism, they didn´t know which neurons were receptors in the olfactory bulb. Part of the reason this mechanism remained such a mystery is the lack of modern technological tools to trigger individual neurons and track their paths.

“One of the challenges with this type of research is that these feedback neurons are not the only neurons that come back to the olfactory bulb,” said Murthy.

“The challenge has always been that there´s no easy way to pick out just one type of neuron to activate.”

To track the pathways of these neurons to the olfactory bulb, Murthy and team employed a virus that had been genetically modified to produce a light-sensitive protein. When this virus was hit with a laser, the proteins lit up, mapping out their pathways.

Murthy explained this high-level of precision allowed his team to determine the tracks of both “principal” neurons and interneurons. The principal neurons are responsible for sending signals to other parts of the brain, but the interneurons play a significant role in formatting the olfactory information as it arrives at the brain.

Without this formatting process, the brain could have difficulty understanding which signals were strong and which were weak.

“If you make a system that is very good at detecting weak signals, it becomes saturated as the signal gets stronger, and eventually it´s impossible to differentiate between strong signals,” said Murthy.

“By inhibiting certain neurons, it ensures that you stay within the detection range, so you don´t miss the weak things, but you don´t miss the very strong things either.”

Earlier studies had found these interneurons in the olfactory bulb are the primary target of such feedback signals. This new study, on the other hand, is the first to show these feedback signals inhibit the activity of the principal neurons.

“When the cortical area decides to send these signals back to the olfactory bulb, it´s effectively turning down the activity of these principal neurons,” said Murthy, in closing,

“Why does the brain do this? Our theory is that the feedback is a way for the cortex to say, ℠I heard you.´ As the olfactory information is sent to higher regions of the brain, these signals come back and turn down the volume on the input.”